1. Field of the Invention
The present invention relates to a method for fabricating semiconductor devices, and more particularly to a method to prevent the formation of a thinner portion of gate oxide at the junction between the side walls and the bottom insulator of a trench type semiconductor device.
2. Description of the prior art
As the design rule continues to shrink, a trench type semiconductor device, for example metal-oxide-semiconductor transistor, has been provided to economize the wafer area.
Referring to FIGS. 1A through 1C, the cross-sectional side views of a conventional method for fabricating a gate oxide within a trench are depicted in sequence.
Referring now to FIG. 1A, a cross-sectional view of the first step is schematically shown. FIG. 1A shows a silicon substrate 10 having a trench 12. A silicon nitride layer 11 is formed overlaying the silicon substrate 10.
Next, as shown in FIG. 1B, a bottom oxide 16 is deposited on the lower portion of the trench 12 by high density plasma chemical vapor deposition (HPCVD).
Referring now to FIG. 1C, a gate oxide layer 18, or example silicon oxide having a thickness of about 500 angstroms, is grown on the side walls of the trench 12.
However, the gate oxide layer 18 is naturally thinner at the junction 20 between the side walls and the bottom oxide 16. This thinner portion will cause junction leakage problems.
In view of the above disadvantages, an object of the invention is to provide a method to prevent the formation of a thinner portion of oxide layer at the junction between the side walls and the bottom oxide.
It is a further object of the invention to provide a method to avoid junction leakage.
The above objects are attained by providing a method to prevent the formation of a thinner portion of insulating layer suitable for a semiconductor substrate having a trench comprising the steps of: (a) forming a pad insulating layer on the side walls and the bottom of said trench; (b) forming a bottom insulator over said pad insulating layer, said bottom insulator filled within said trench; (c) etching the upper portion of said bottom insulator to form a bottom insulator having a concave surface; and (d) forming an insulating layer on the side walls of said trench.
The above objects are also attained by providing a method to prevent the formation of a thinner portion of oxide layer suitable for a semiconductor substrate having a trench comprising the steps of: (a) forming a bottom insulator having a concave surface on the bottom of said trench; and (b) forming an oxide layer on the side walls of said trench.
An aspect of the invention is to provide a method to prevent the formation of a thinner portion of insulating layer in which said insulating material is oxide material.
Another aspect of the invention is to provide a bottom insulator (oxide) which has a specially shaped surface, especially concave shaped, for growing a conformal gate oxide layer.